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US military officials said the purpose of the deal was not immediately clear. But as a result, US military advisers could soon find themselves in close proximity to Syrian and Russian troops. The advisers are currently training local forces in Manbij to combat ISIS in the vicinity, part of the approximate 500-strong contingent of American Special Operations Forces in Syria.

A spokesman for the US-led military coalition confirmed Saturday that additional international troops had been deployed in and around Manbij after images of coalition armored vehicles appeared online.

Col. John Dorrian wrote on Twitter that the coalition "has taken this deliberate action to reassure coalition members and partner forces, deter aggression and keep focus on defeating ISIS."

Dorrian added that the coalition has maintained a presence there since the city's recapture from ISIS.

The Manbij Military Council has safe-guarded the town following the defeat of ISIS there in August by the Syrian Democratic Forces, a mix of Arab and Kurdish fighters that are backed by US and coalition air power and make up part of the council. The SDF has largely sought to avoid conflict with Damascus and its Russian backers, focusing primarily on ISIS.

The town is considered to be of immense tactical value due to its proximity to supply routes into and out of Syria. Its capture was hailed by US commanders as a major victory.

Russia's ability to strike a deal with one of America's closest allies in the fight against ISIS, however, could be viewed as a sign of eroding US influence in Syria. It could also lay the ground work for further cooperation between Washington and Moscow in the civil-war-wracked country, something President Donald Trump has expressed openness to in the past.

The Russian Ministry of Defense confirmed the move Friday, saying in a statement on Facebook that Syrian troops entered the area March 3 following the signing of the agreement.

US Navy Capt. Jeff Davis said the development "has not changed anything we are doing, which is we continue to focus on defeating ISIS."

But the battle space has already proven congested, with Russian warplanes accidentally striking US-backed rebels as US troops were some three miles away, according to Lt. Gen. Stephen Townsend, commander of the US-led counter-ISIS coalition.

"Essentially three armies and an enemy force have all converged within the same grid square. It's very difficult and complicated," he said.

The new agreement is likely to further complicate the picture in Northern Syria, increasing the potential for an armed clash between America's NATO ally Turkey and the combined Russia and Syrian regime forces.

The Syrian Defense Force's role in the Manbij battle against ISIS irked Turkey, which sees the group as closely aligned with the PKK, a Kurdish separatist group that has carried out terrorist attacks against Turkey. The Pentagon does not consider the two groups linked.

The Turkish government warned Tuesday that it would advance on Manbij because of the presence of the Kurdish fighters, though US military officials said they have long left the town.

The agreement between the Manbij Council and Moscow could be aimed at deterring a Turkish advance by placing Russian and Syrian troops between the town and the Turkish military. A US defense official told CNN, however, that there weren't any visible indications that Turkey was launching such an offensive.

But the new deployment of US troops with the stated aim of deterring "hostile acts" could be seen as an additional attempt to prevent any clashes between America's allies.

Dorrian tweeted the coalition "increased force presence in, around Manbij to deter hostile acts, enhance governance and ensure there's no persistent YPG presence," referring to the US-backed Kurdish militia that has been an ally in the fight against ISIS but is strongly opposed by Turkey.

"We encourage all forces to remain focused on the counter-ISIS fight and concentrate their efforts on defeating ISIS and not towards other objectives," Eric Pahon, a defense department spokesman, added.